A vehicle body frame is known that is divided into a front frame portion including a head pipe portion and a main frame portion, and a rear frame portion including a pivot plate portion. The frame portions are formed integrally by a hollow casting, whereby the number of welding points are reduced thereby reducing a manufacturing cost. See, for example, JP-A-2004-114719.
In the structure known in the related art, the parts manufactured by the hollow casting are relatively large. Thus, it is preferable to divide the front frame portion into the head pipe portion and the main frame portion.
In this case, it is necessary to secure a sufficient welding length for increasing a joint strength between the head pipe portion and the main frame portion. In the case wherein an opening is formed in the vicinity of the joint portion, a structure is desirable wherein the welding length can be secured without applying welding on the inner side of the opening.